1. Field of the Invention
The present invention relates generally to borehole logging apparatus and, more particularly, to an improved borehole logging tool for detecting nuclear radiation by use of a cryogenically cooled semiconductor detector.
2. The Prior Art
In many instances, the exploration of subsurface earth formations for oil or the like is best achieved by gamma ray analysis. This, however, frequently requires the use of high resolution, semiconductor detectors, such as a high purity germanium (HPGe) detector or a lithium-drifted germanium (Ge(Li)) detector, which operate reliably only at cryogenic temperatures, e.g. on the order of -180.degree. C. The harsh ambient temperatures typically encountered in borehole logging, e.g., up to and above 200.degree. C., have therefore been a serious obstacle to the utilization of semiconductor detectors in borehole exploration.
Good thermal insulation is essential for the proper operation of semiconductor detectors at elevated borehole temperatures. Size and other limitations inherent in borehole logging sondes, however, have hindered the provision and maintenance of good thermal insulation in a borehole environment. Consequently, prior art logging tools employing semiconductor detectors have had unduly short holding or operating times or have suffered from other deficiencies which have impaired their usefulness. Although these deficiencies of prior borehole devices have been largely overcome in the laboratory, the laboratory techniques utilized for this purpose, e.g. control of ambient temperatures by refrigeration, large-diameter cryostats, high volume vacuum pumps, are not applicable or cannot be readily adapted to use in a borehole sonde.